The present invention relates to the structure of a semiconductor laser device which is suitable for use as a Fabry-Perot semiconductor laser device or a semiconductor laser device with the DBR structure. More particularly, the present invention relates to an improvement in optical output power of the semiconductor laser device.
Conventionally, writable and rewritable optical disk apparatuses have been required for writing at higher speeds and this in turn has demanded an even greater power output of an infrared semiconductor laser device. To provide a semiconductor laser device with a greater power output, it is necessary to reduce the risk of catastrophic optical damage (hereinafter referred to as the COD) to the edges of the laser device and improve the level of output saturation resulting from heat.
As a first prior-art example of the most typical semiconductor laser device, a semiconductor laser device is known which comprises the active layer of GaAs and two cladding layers of AlxGa1−xAs (0≦x≦1, hereinafter referred to as AlGaAs in some cases) for sandwiching the active layer therebetween vertically.
As a second prior-art example, also known is a semiconductor laser device comprising the active layer of GaAs, Alx1Ga1−x1As (0≦x1≦1, hereinafter referred to as AlGaAs in some cases), or Inx2Ga1−x2As (0≦x2≦1, hereinafter referred to as InGaAs in some cases). The laser device also comprises two cladding layers of (Alx3Ga1−x3)yIn1−yP (0≦x3≦1, 0≦y≦1, hereinafter referred to as AlGaInP in some cases) which have a large bandgap and sandwich the active layer therebetween vertically. (For example, refer to Japanese Patent Laid-Open Publication No. Hei 5-218582.)
However, the aforementioned semiconductor laser devices have disadvantages that should be improved as described below.
For example, under conditions for generating laser of the semiconductor laser device at high output powers, a considerable increase in temperature in the active layer would cause an excessive density of carriers injected to the active layer, resulting in spillover (overflow) of carriers from the active layer to the cladding layers. The spillover of carriers from the active layer to the cladding layers causes the carriers to be dissipated for non-radiative recombination, thereby causing a further increase in temperature in the active layer. Consequently, under conditions for generating laser at high output powers, a phenomenon of thermal saturation occurs in which the carriers are dissipated due to an increase in temperature of the chip and this causes the optical output not to exceed a certain value with an increase in current. In particular, a semiconductor laser device having a single quantum well layer is susceptible to this phenomenon.
In addition, under conditions for generating laser of the semiconductor laser device at high output powers, a considerable increase in temperature in the active layer causes a considerable increase in temperature of the resonator cavity end faces. In some cases, this causes the so-called COD to occur in which light is increasingly absorbed at the resonator cavity end faces and this causes a local damage (melting) to the crystal structure when a certain amount of current is achieved.
The aforementioned phenomenon of thermal saturation or the COD has been an impediment to the improvement of efficiency of the semiconductor laser device.
On the other hand, suppose that a crystal containing no phosphor, for example, an AlGaAs layer is grown after the crystal growth of a compound semiconductor containing phosphor such as an AlGaInP layer. In this case, a gas containing phosphor is decomposed into phosphor, which would contaminate the AlGaAs layer. Accordingly, an AlGaAs layer employed for the active layer of a light-emitting element would cause the characteristics of the active layer to change due to the contamination of the AlGaAs layer with the phosphor. This would make it difficult to control the efficiency and the wavelength for generating laser of the semiconductor laser device, thereby raising the possibility of reducing the manufacturing yield of the semiconductor laser device.